The semiconductor manufacturing industry uses small cleanroom enclosures ("mini-enclosures") for many different kinds of manufacturing operations. Inside the mini-enclosures are process tools such as photo process, stepper, ebeam, measurement, wet station, CMP, and others. The manufacturing operations require high precision and have a low tolerance to contaminants. It is necessary to maintain constant environmental conditions inside the enclosure and minimize variations in temperature, humidity, volumetric airflow, etc. It is much easier to precisely control the environment inside a mini-enclosure because of its smaller interior space or volume. For this reason, mini-enclosures are now used throughout the semiconductor manufacturing industry.
Stand-alone environmental control units are typically used to control mini-enclosure temperature, humidity, and air flow rates. This is illustrated in FIG. 3, which shows a typical prior art stand-alone installation at 1.
The unit 1 is physically spaced apart from a mini-enclosure 2 at a distance that will vary from one manufacturing facility to the next. The unit 1 draws surrounding ambient air, cools or heats it, as necessary, and adjusts its humidity. The volumetric flow rate through the unit 1 is controlled by an adjustment to the fan 3.
As shown at 5, conditioned air is driven by the fan 3 through a duct 6 and filter 7, and then into the mini-enclosure 2. Temperature and humidity sensors 8 are positioned at or near the point of airflow entry into the enclosure. The sensors provide continual feedback to a controller (indicated generally at 9) in the stand-alone unit 1. The controller 9 causes the unit to adjust temperature and humidity based on sensor feedback.
The ambient air temperature outside the duct 6 can be significantly different from the temperature of the conditioned air 5. Also, even though the system is typically located in a controlled, air-conditioned space inside a building, the ambient air temperature can vary significantly. Heat transfer from the ambient air can alter the temperature and relative humidity of the air 5 as it exits the unit 1 and travels through the duct 6. Consequently, while it is possible to control airflow conditions at the point where the conditioned air 5 exits the unit 1 and enters the duct 6, the impact of the duct introduces inconsistent and uncontrollable variations in the airflow. The longer the duct 6, the greater the problem.
The physical dimensions and weight of the typical unit 1 make it impossible, as a practical matter, to eliminate the duct 6 by mounting the unit directly on top of an enclosure 2. The stand-alone unit 1 has its own internal air conditioning unit which cools or otherwise sets the temperature of the air 5. In part, the size and weight of the internal air conditioning unit limits the ability to make design changes to the unit 1.
Moreover, the internal air conditioning unit uses an operating fluid to produce conditioned air via a conventional refrigeration cycle. The fluid undergoes a phase change during compression and expansion portions of the cycle. The different portions of the cycle are never absolutely constant which produces rapid temperature fluctuations within a narrow range. What this means is that working fluid temperatures produced by the phase change are difficult to maintain at a consistent level with ultra-high precision.
The present invention addresses the above drawbacks and provides certain improvements. Among other things, the invention eliminates the internal air conditioning unit described above. This, in turn, allows a much smaller environmental control unit to be designed. In fact, the environmental control unit disclosed here can be reduced in size so that a typical mini-enclosure can physically support the weight of the unit. This permits direct mounting of the unit to the enclosure and moves the point of temperature and humidity control to the point of air entry into the enclosure. The invention also provides a better system for producing highly stable temperature and humidity control.